Ultrasonic multiplexing network for implantable medical devices

ABSTRACT

A system and method for transmitting data ultrasonically through biological tissue employs a network of a plurality of nodes, at least a portion of the nodes implantable within the biological tissue. At least one implanted node includes a transmitter having an orthogonal frequency division multiplex signal generator to encode an ultrasonic signal for transmission through the biological tissue to an ultrasonic receiver at another node.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 120 of U.S. patentapplication Ser. No. 15/622,492, filed on Jun. 14, 2017, entitled“Ultrasonic Multiplexing Network for Implantable Medical Devices,” whichclaims priority under 35 U.S.C. § 119(e) of U.S. Provisional ApplicationNo. 62/349,711, filed on Jun. 14, 2016, entitled “UltrasonicMultiplexing Network for Implantable Medical Devices,” and which alsoclaims priority under 35 U.S.C. § 120 of International Application No.PCT/US2016/012439, filed on Jan. 7, 2016, entitled “UltrasonicMultiplexing Network for Implantable Medical Devices,” which claimspriority under 35 U.S.C. § 119(e) of U.S. Provisional Application No.62/100,628, filed on Jan. 7, 2015, entitled “Multi-Carrier UltrasonicCommunications, Resource Allocation, and Medium Access for ImplantableDevices,” the disclosures of all of which are hereby incorporated byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was developed with financial support from Grant No.CNS-1253309 from the National Science Foundation. The U.S. Governmenthas certain rights in the invention.

BACKGROUND

Implantable medical sensing and actuating devices with wirelesscapabilities are used in many digital health applications. Existingwireless medical implants are connected through radio frequency (RF)electromagnetic waves. RF-based solutions tend to scale down traditionalwireless technologies, such as Wi-Fi or Bluetooth, to the intrabodyenvironment, with little or no attention paid to the peculiarcharacteristics and safety requirements of the human body and to theprivacy and security requirements of patients.

SUMMARY OF THE INVENTION

The invention relates to an implantable network of medical sensing andactuating devices that communicate via an ultrasonic communicationsystem and method. The system and method employ an orthogonal frequencydivision multiplexing (OFDM) scheme to offer link-to-link physical layeradaptation, with distributed control to enable multiple access amonginterfering implanted devices. The data rate of the transmitters in thenetwork can be adapted to a current level of interference bydistributively optimizing the physical layer parameters.

Other aspects of the method and system include the following:

1. A system for transmitting data ultrasonically through biologicaltissue comprising:

a network comprising a plurality of nodes, at least a portion of thenodes implantable within a body;

a first node implantable in the body and comprising an ultrasonictransducer and a transmitter, and a second node comprising an ultrasonicreceiver;

the transmitter at the first node including an orthogonal frequencydivision multiplex (OFDM) signal generator operative to encode an inputinformation bit stream on orthogonal subcarriers for transmission as anultrasonic signal through the body to the ultrasonic receiver at thesecond node; and

the ultrasonic receiver at the second node operative to decode theultrasonic signal received from the first node to recover theinformation bit stream.

2. The system of embodiment 1, wherein the OFDM signal generator isoperative to generate a baseband modulated signal as a sum over a numberof subcarriers of the symbols to be transmitted as a function of afrequency spacing between the subcarriers for a time block of a givenduration.3. The system of embodiment 2, further comprising introducing a guardtime between time blocks.4. The system of embodiment 3, wherein the guard time comprises silenceor a repetition of the time block.5. The system of embodiment 2, further comprising a symbol mapper to mapthe bit stream into a constellation of a modulation scheme.6. The system of embodiment 5, wherein the modulation scheme comprisesM-phase-shift-keying or M-quadrature-amplitude-modulation.7. The system of any of embodiments 1-6, wherein the OFDM signalgenerator comprises:

a serial to parallel convertor to convert the input information bitstream into a plurality of parallel data strings,

an inverse Fourier transformer to generate a frequency domainrepresentation of the input information bit stream,

a parallel to serial converter for converting the frequency domainrepresentation into a serial stream, and

an up-converter to convert the signal to a carrier frequency fortransmission through the biological tissue.

8. The system of any of embodiments 1-7, wherein the receiver comprises:

a down-converter to convert the received signal to a baseband signal,

a serial to parallel converter to convert the baseband signal into aplurality of parallel data strings,

a Fourier transformer; and

a parallel to serial converter to convert the parallel data strings intoa serial data string.

9. The system of any of embodiments 1-8, wherein the OFDM signalgenerator is operative to send symbols on a set of occupied subcarrierscomprising a subset of available subcarriers.

10. The system of embodiment 9, wherein the occupied subcarriers arefixed or selected randomly and change in consecutive blocks within aframe.

11. The system of embodiment 10, wherein the occupied subcarriers areselected by a pseudo-random frequency-hopping sequence generated byseeding a random number generator with an identification unique to thetransmitter.

12. The system of embodiment 9, wherein the occupied subcarriers have afixed frequency.

13. The system of embodiment 9, wherein the occupied subcarriers areselected randomly and change in consecutive blocks within a frame.

14. The system of embodiment 13, wherein the occupied subcarriers areselected by a pseudo-random frequency-hopping sequence generated byseeding a random number generator with an identification unique to thetransmitter.

15. The system of any of embodiments 1-14, wherein the OFDM signalgenerator is operative to send symbols in blocks at fixed or randomlyselected time chips within a time frame.

16. The system of embodiment 15, wherein the OFDM signal generator isoperative to send symbols according to a pseudo-random time hoppingsequence generated by seeding a random number generator with anidentification unique to the transmitter.

17. The system of any of embodiments 1-14, wherein the OFDM signalgenerator is operative to send symbols in clocks at fixed time chipswithin a time frame.

18. The system of any of embodiments 1-14, wherein the OFDM signalgenerator is operative to send symbols in blocks at randomly selectedtime chips within a time frame.

19. The system of embodiment 18, wherein the OFDM signal generator isoperative to send symbols according to a pseudo-random time hoppingsequence generated by seeding a random number generator with anidentification unique to the transmitter.

20. The system of embodiment 19, wherein the receiver is operative todecode the ultrasonic signal at the receiver by seeding a randomgenerator with the identification of the transmitter to generate thesame pseudo-random time hopping sequence.

21. The system of any of embodiments 1-20, wherein the OFDM signalgenerator is operative to provide forward error correction.

22. The system of embodiment 21, wherein the forward error correctioncomprises adding t parity symbols to k information symbols to make an nsymbol block.

23. The system of embodiment 22, wherein the forward error correctioncomprises the addition of parity symbols using a block code or aconvolutional code.

24. The system of embodiment 23, wherein the block code comprises aReed-Solomon code.

25. The system of any of embodiment 1-24, wherein the OFDM signalgenerator is operative to provide one or more modulation techniques at asubcarrier level, a block level, or a frame level.

26. The system of embodiment 25, wherein the modulation technique isselected to optimize a data rate as a function of one or more of anumber of occupied subcarriers, a number of time chips per time frame,an error correction coding rate, and a modulation rate.27. The system of any of embodiments 1-26, wherein the receiver isoperative to detect an incoming frame from the transmitter and toidentify a starting point of a packet.28. The system of embodiment 27, wherein identifying the starting pointcomprises correlating the received ultrasonic signal with a local copyof a preamble preceding each OFDM frame.29. The system of embodiment 28, wherein the preamble comprises a pseudonoise sequence or a chirp sequence.30. The system of any of embodiments 1-29, wherein the receiver isoperative to determine a signal to interference-plus-noise ratio as afunction of instantaneous power, time-hopping frame length, and numberof occupied subcarriers.31. The system of any of embodiments 1-30, wherein the receiver isoperative to maximize a transmission rate between the transmitter andthe receiver by selecting an instantaneous power, a number of occupiedsubcarriers, a time-hopping frame length, a forward error correctioncoding rate and a modulation rate based on a level of interference andchannel quality measured at the receiver and on a level of interferencegenerated by the receiver in communications to other nodes.32. The system of any of embodiments 1-31, wherein the receiver isoperative to determine an instantaneous power value, a number ofoccupied subcarriers, a time-hopping frame length, a forward errorcorrection coding rate and a modulation rate that maximizes a data rate.33. The system of embodiment 32, wherein the receiver is furtheroperative to maximize the data rate subject to a signal tointerference-plus-noise ratio per node being above a minimum value and adata rate per node being above a minimum value.34. The system of any of embodiments 1-33, wherein the receiver isfurther operative to determine an energy rate, the energy ratecomprising an energy per bit or an average power radiated per second.35. The system of embodiment 34, wherein the receiver is furtheroperative to minimize the energy rate subject to a signal tointerference-plus-noise ratio per node being above a minimum value and adata rate per node being above a minimum value.36. The system of any of embodiments 1-35, wherein the transmitter isoperative to open communication to a receiver on a common controlchannel using a two-way hand-shake procedure, and, after receiving aclear-to-transmit signal from the receiver, to transmit on a dedicatedchannel to the receiver a frequency-hopping sequence and a time-hoppingsequence, and the receiver is operative to transmit to the transmitteran optimal transmission strategy.37. The system of embodiment 36, wherein the transmitter is operative todetermine the frequency-hopping sequence and the time-hopping sequenceby seeding a random number generator with an identification unique tothe transmitter.38. The system of any of embodiments 36-37, wherein the optimaltransmission strategy comprises a number of occupied subcarriers, atime-hopping frame length, a forward error correction coding rate, and amodulation rate.39. The system of any of embodiments 36-38, wherein the receiver isoperative to exchange information regarding a level of tolerableinterference over the common control channel with other receiving nodes.40. The system of any of embodiments 1-39, wherein the first nodefurther comprises an ultrasonic receiver to decode an ultrasonic signalreceived from another node of the plurality of nodes.41. The system of any of embodiments 1-40, wherein the second nodefurther comprises an ultrasonic transducer and a transmitter, thetransmitter at the second node including an orthogonal frequencydivision multiplex (OFDM) signal generator operative to encode an inputinformation bit stream on orthogonal subcarriers for transmission as anultrasonic signal through the body to the first node or to another nodeof the plurality of nodes or to all of the nodes of the plurality ofnodes.42. The system of any of embodiments 1-41, wherein all of the nodes ofthe plurality of nodes comprise a transmitter including an orthogonalfrequency division multiplex (OFDM) signal generator operative to encodean input information bit stream on orthogonal subcarriers fortransmission as an ultrasonic signal through the body and an ultrasonicreceiver to decode an incoming ultrasonic signal to recover aninformation bit stream.43. The system of any of embodiments 1-42, wherein the first node andthe second node are operable at a data rate of at least 28 Mbit/s.44. The system of any of embodiments 1-43, wherein the second node isimplantable within the body.45. The system of any of embodiments 1-44, wherein the second node is anendoscopic pill.46. The system of any of embodiments 1-43, wherein the second node isdisposed outside of the body.47. The system of any of embodiments 1-46, wherein the biological tissueis human tissue or non-human animal tissue.48. The system of any of embodiments 1-47, wherein the first implantablenode further comprises a sensor operative to sense one or morebiological parameters.49. The system of embodiment 48, wherein the sensor is selected from thegroup consisting of a cardiac rhythm monitor, a pulse monitor, a bloodpressure sensor, a glucose sensor, a drug pump monitor, a neurologicalsensor, a motion sensor, a gyroscope, an accelerometer, a sleep sensor,a REM sleep duration sensor, a still camera, a video camera, a sensorfor one or more biomolecules, a sensor for one or more pharmaceuticalagents or pharmaceutical formulation ingredients, and a sensor for adissolved gas or ion, or for pH, ionic strength, or osmolarity.50. The system of embodiment 49, wherein the sensor for one or morebiomolecules comprises a sensor for one or more peptides, oligopeptides,polypeptides, proteins, glycoproteins, antibodies, antigens, nucleicacids, nucleotides, oligonucleotides, polynucleotides, sugars,disaccharides, trisaccharides, oligosaccharides, polysaccharides,lipids, glycolipids, proteolipids, cytokines, hormones,neurotransmitters, metabolites, glycosaminoglycans, and proteoglycans.51. The system of any of embodiments 1-50, wherein the first implantablenode further comprises an actuator.52. The system of embodiment 51, wherein the actuator is selected fromthe group consisting of a drug pump, a heart stimulator, a heartpacemaker, a bone growth stimulator,

and a neuromuscular electrical stimulator.

53. The system of any of embodiments 1-52, wherein at least two of theplurality of nodes are implantable within a body.

54. The system of any of embodiments 1-53, wherein at least three of theplurality of nodes are implantable within a body.

55. The system of embodiments 1-54, wherein at least a portion of thenodes are implanted in the body.

56. The system of embodiments 1-55, wherein all of the nodes of theplurality of nodes are implantable within the body.

57. A method for transmitting data ultrasonically through biologicaltissue comprising:

at a first node implanted in a body, encoding an information bit streamon orthogonal subcarriers using an orthogonal frequency divisionmultiplexing modulation scheme;

transmitting the encoded signal through biological tissue; and

at a second node, receiving the encoded signal and decoding the signalto recover the information bit stream.

58. The method of embodiment 57, encoding the information bit streamcomprises generating a baseband modulated signal as a sum over a numberof subcarriers of the symbols to be transmitted as a function of afrequency spacing between the subcarriers for a time block of a givenduration.59. The method of embodiment 58, further comprising introducing a guardtime between time blocks.60. The method of embodiments 59, wherein the guard time comprisessilence or a repetition of the time block.61. The method of embodiment 58, further comprising mapping the bitstream into a constellation of a modulation scheme.62. The method of embodiment 61, wherein the modulation scheme comprisesM-phase-shift-keying or M-quadrature-amplitude-modulation.63. The method of any of embodiments 57-62, further comprising, at thefirst node:

serial-to-parallel converting the input information bit stream into aplurality of parallel data strings,

generating a frequency domain representation of the input informationbit stream by an inverse Fourier transform,

parallel-to-serial converting the frequency domain representation into aserial stream, and

up-converting the signal to a carrier frequency for transmission throughthe biological tissue.

64. The method of any of embodiments 57-63, further comprising, at thesecond node:

down-converting the received signal to a baseband signal,

serial-to-parallel converting the baseband signal into a plurality ofparallel data strings,

Fourier transforming the plurality of data strings; and

parallel-to-serial converting the parallel data strings into a serialdata string.

65. The method of any of embodiments 57-64, further comprising sendingsymbols on a set of occupied subcarriers comprising a subset ofavailable subcarriers.

66. The method of embodiment 65, wherein the occupied subcarriers arefixed or selected randomly and change in consecutive blocks within aframe.

67. The method of embodiment 66, wherein the occupied subcarriers areselected by a pseudo-random frequency-hopping sequence generated byseeding a random number generator with an identification unique to thefirst node.

68. The method of embodiment 65, wherein the occupied subcarriers arefixed.

69. The method of embodiment 65, wherein the occupied subcarriers areselected randomly and change in consecutive blocks within a frame.

70. The method of embodiment 69, wherein the occupied subcarriers areselected by a pseudo-random frequency-hopping sequence generated byseeding a random number generator with an identification unique to thefirst node.

71. The method of any of embodiments 57-70, further comprising sendingsymbols in blocks at fixed or randomly selected time chips within a timeframe.

72. The method of embodiment 71, further comprising sending symbolsaccording to a pseudo-random time hopping sequence generated by seedinga random number generator with an identification unique to thetransmitter.

73. The method of any of embodiments 57-70, further comprising sendingsymbols in blocks at fixed time chips within a time frame.

74. The method of any of embodiments 57-70, further comprising sendingsymbols in blocks at randomly selected time chips within a time frame.

75. The method of embodiment 74, further comprising sending symbolsaccording to a pseudo-random time hopping sequence generated by seedinga random number generator with an identification unique to thetransmitter.

76. The method of any of embodiments 71 and 74, further comprising, atthe second node, decoding the ultrasonic signal by seeding a randomgenerator with the identification of the first node to generate the samepseudo-random time hopping sequence.

77. The method of any of embodiments 57-76, further comprising providingforward error correction.

78. The method of embodiment 77, wherein the forward error correctioncomprises adding t parity symbols to k information symbols to make an nsymbol block.

79. The method of embodiment 78, wherein the forward error correctioncomprises the addition of parity symbols using a block code or aconvolutional code.

80. The method of embodiment 79, wherein the block code comprises aReed-Solomon code.

81. The method of any of embodiments 57-80, further comprising providingone or more modulation techniques at a subcarrier level, a block level,or a frame level.

82. The method of embodiment 81, wherein the modulation technique isselected to optimize a data rate as a function of one or more of anumber of occupied subcarriers, a number of time chips per time frame,an error correction coding rate, and a modulation rate.83. The method of any of embodiments 57-82, further comprising, at thesecond node, detecting an incoming frame from the first node andidentifying a starting point of a packet.84. The method of embodiment 83, wherein identifying the starting pointcomprises correlating the received ultrasonic signal with a local copyof a preamble preceding an OFDM frame.85. The method of embodiment 84, wherein the preamble comprises a pseudonoise sequence or a chirp sequence.86. The method of any of embodiments 57-85, further comprising, at thesecond node, determining a signal to interference-plus-noise ratio as afunction of instantaneous power, time-hopping frame length, and numberof occupied subcarriers.87. The method of any of embodiments 57-86, further comprising, at thesecond node, maximizing a transmission rate between the transmitter andthe receiver by selecting an instantaneous power, a number of occupiedsubcarriers, a time-hopping frame length, a forward error correctioncoding rate and a modulation rate based on a level of interference andchannel quality measured at the receiver and on a level of interferencegenerated by the second node in communications to other nodes.88. The method of any of embodiments 57-87, further comprising, at thesecond node, determining an instantaneous power value, a number ofoccupied subcarriers, a time-hopping frame length, a forward errorcorrection coding rate and a modulation rate that maximizes a data rate.89. The method of embodiment 88, further comprising, at the second node,maximizing the data rate subject to a signal to interference-plus-noiseratio per node being above a minimum value and a data rate per nodebeing above a minimum value.90. The method of any of embodiments 57-89, further comprising, at thesecond node, determining an energy rate, the energy rate comprising anenergy per bit or an average power radiated per second.91. The method of embodiment 90, further comprising, at the second node,minimizing the energy rate subject to a signal tointerference-plus-noise ratio per node being above a minimum value and adata rate per node being above a minimum value.92. The method of any of embodiments 57-91, further comprising, at thefirst node, opening communication to the second node on a common controlchannel using a two-way hand-shake procedure, and, after receiving aclear-to-transmit signal from the second node, transmitting on adedicated channel to the second node a frequency-hopping sequence and atime-hopping sequence, and at the second node, transmitting to the firstnode an optimal transmission strategy.93. The method of embodiment 92, further comprising, at the first node,determining the frequency-hopping sequence and the time-hopping sequenceby seeding a random number generator with an identification unique tothe transmitter.94. The method of any of embodiments 92-93, wherein the optimaltransmission strategy comprises a number of occupied subcarriers, atime-hopping frame length, a forward error correction coding rate, and amodulation rate.95. The method of any of embodiments 57-94, further comprising, at thesecond node, exchanging information regarding a level of tolerableinterference over the common control channel with other receiving nodes.96. The method of any of embodiments 57-95, wherein the first node andthe second node are operable at a data rate of at least 28 Mbit/s.97. The method of any of embodiments 57-96, wherein the second node isimplanted within the body.98. The method of any of embodiments 57-97, wherein the second node isan endoscopic pill.99. The method of any of embodiments 57-98, wherein the second node isdisposed outside of the body.100. The method of any of embodiments 57-99, wherein the biologicaltissue is human tissue or non-human animal tissue.101. The method of any of embodiments 57-100, further comprisingreceiving at the first node an encoded ultrasonic signal and decodingthe signal to recover an information bit stream.102. The method of any of embodiments 57-101, further comprising sensingat one or more of the implantable nodes one or more biologicalparameters sensed by a sensor.103. The method of embodiment 102, wherein the sensor is selected fromthe group consisting of a cardiac rhythm monitor, a pulse monitor, ablood pressure sensor, a glucose sensor, a drug pump monitor, aneurological sensor, a motion sensor, a gyroscope, an accelerometer, asleep sensor, a REM sleep duration sensor, a still camera, and a videocamera.104. The method of any of embodiments 57-103, further comprisingactuating at one or more of the implantable nodes an actuator.105. The method of embodiment 104, wherein the actuator is selected fromthe group consisting of a drug pump, a heart stimulator, a heartpacemaker, a bone growth stimulator,

and a neuromuscular electrical stimulator.

DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic diagram of an OFDM encoder (top) and decoder(bottom);

FIG. 2 is a schematic diagram of an example of pure time-hoppingstrategy (top), and a combined frequency- and time-hopping strategy(bottom);

FIG. 3 is a schematic illustration of a network of nodes implanted in abody;

FIG. 4 is a block diagram of a prototype ZP-OFDM transmitter (top) andreceiver (bottom);

FIG. 5 is a schematic diagram of hardware architecture of a prototypeSDR-based node;

FIG. 6 is an illustration of SDR-based ultrasonic nodes communicatingthrough a human-kidney phantom;

FIG. 7 is a further illustration of SDR-based ultrasonic nodescommunicating through a human-kidney phantom; and

FIG. 8 is a graph of BER versus SNR results for different modulationschemes.

DETAILED DESCRIPTION OF THE INVENTION

Radio frequency (RF) technology presents several limitations that cannegatively affect patients' medical experience and safety. First, RFwaves do not propagate well in biological tissues, which leads to higherenergy consumption and heating of the tissues. Second, the RF frequencyspectrum is scarce, strictly regulated, and already crowded with manydevices interfering with one another. Therefore, RF-based technologiesraise serious concerns about potential interference from existing RFcommunication systems that can unintentionally undermine the reliabilityand security of an intra-body network, and ultimately the safety of thepatient. Third, RF communications can be easily jammed, i.e.,intentionally disrupted by artificially generated interference, oreavesdropped by malicious agents. This raises major privacy and securityred flags for intra-body networks, and a risk for the patient. Fourth,the medical community is still divided on the risks caused by continuousexposure of human tissues to RF radiation. Therefore, a massivedeployment of RF implantable devices may represent a potential risk forthe patient. Finally, the dielectric nature of the human body alsoaffects the coupling between on-body RF antennas and the body itself. Inparticular, the gain and the radiation pattern of the antennadeteriorate because of the contact or proximity with the human body,while the resonant frequency and the input impedance of the antenna mayshift from their nominal values.

Accordingly, a communications system is described herein that usesultrasonic waves as an alternative carrier of information in biologicaltissues. Ultrasonic waves are acoustic waves with frequency higher thanthe upper threshold for human hearing, i.e., generally 20 kHz. In someembodiments, the communications system can use near-ultrasonic waves,such as greater than 17 kHz. The ultrasonic communications systememploys an orthogonal frequency division multiplexing (U-OFDM)-basednetworking scheme that offers link-to-link physical layer adaptation,with distributed control to enable multiple access among interferingimplanted devices. U-OFDM is based on the idea of regulating the datarate of each transmitter to adapt to the current level of interferenceby distributively optimizing the physical layer parameter.

1. Ultrasonic Intra-Body Communications

Ultrasounds are mechanical waves that propagate in an elastic medium atfrequencies above the upper limit for human hearing, i.e., 20 kHz.

Attenuation.

Two main mechanisms contribute to ultrasound attenuation in tissues,i.e., absorption and scattering. An initial pressure P₀ decays at adistance dP(d)=P ₀ e ^(−ad),  (1)where α (in [Np·cm⁻¹]) is an amplitude attenuation coefficient thatcaptures all the effects that cause dissipation of energy from theultrasound wave. Parameter a depends on the carrier frequency throughα=af^(b), where f represents the carrier frequency (in MHz) and a (in[Np m⁻¹ MHz^(−b)]) and b are attenuation parameters characterizing thetissue.

Propagation Speed.

Ultrasonic wave propagation is affected by propagation delays that areorders of magnitude higher than RF. The propagation speed of acousticwaves in biological tissues is approximately 1500 m/s, as compared to2×10⁸ m/s for RF waves.

Operating Frequency.

Considerations in determining the operating frequency are (i) thefrequency dependence of the attenuation coefficient, and (ii) thefrequency dependence of the beam spread of ultrasonic transducers (whichis inversely proportional to the ratio of the diameter of the radiatingsurface and the wavelength). Therefore, higher frequencies help keep thetransducer size small, but result in higher signal attenuation. Sincemost biomedical sensing applications require directional transducers,one needs to operate at the lowest possible frequencies compatible withsmall-size transducers and required signal bandwidth. For propagationdistances in the order of several cm. the operating frequency should notexceed 10 MHz.

Reflections and Scattering.

The human body is composed of different organs and tissues withdifferent sizes, densities and sound speeds. Therefore, it can bemodeled as an environment with pervasive presence of reflectors andscatterers. The direction and magnitude of the reflected wave depend onthe orientation of the boundary surface and on the acoustic impedance ofthe tissues, while scattered reflections occur when an acoustic waveencounters an object that is relatively small with respect to itswavelength or a tissue with an irregular surface. Consequently, thereceived signal is obtained as the sum of numerous attenuated, possiblydistorted, and delayed versions of the transmitted signal.

2. Ultrasonic Orthogonal Frequency Division Multiplexing (U-OFDM)

OFDM.

Orthogonal frequency division multiplexing (OFDM) uses a large number ofclosely spaced orthogonal subcarriers, such that for each subcarrier thechannel is subject to flat fading. In the time domain, this comprisesdividing a high data rate stream into multiple low rate streams, eachtransmitted on a different subcarrier. In this way the symbol rate oneach subcarrier is reduced, and hence the effect of intersymbolinterference (ISI) caused by multipath delay spread is reduced. In eachsub-carrier a conventional modulation scheme can be used, e.g.,M-Phase-Shift-Keying (PSK) and M-Quadrature-Amplitude-Modulation (QAM).OFDM offers high spectral efficiency and robustness against narrow-bandco-channel interference, intersymbol interference (ISI) and multipathfading. Assume a bandwidth B divided in a set.

of subcarriers, |F| being the number of subcarriers. Assume |F| symbolsto be transmitted X_(k), with k=1, . . . , |F|. The |F| symbols can bedrawn from any of the constellation available, e.g., X_(k)={−1, 1} for aBPSK modulation. The OFDM baseband modulated signal is given by

$\begin{matrix}{{{x(t)} = {\sum\limits_{k = 0}^{{\mathcal{F}} - 1}\;{X_{k}e^{2\;\pi\; k\; f_{s}t}}}},{t \in \left\lbrack {0,T_{B}} \right\rbrack}} & (2)\end{matrix}$where f_(s) is the frequency spacing between subcarriers. The expressionabove represents an OFDM block of duration TB, where each symbol X_(k)is transmitted on the k^(th) subcarrier. If F is selected equal to 1,than the U-OFDM node will transmit in a single-carrier fashion, e.g.,traditional narrowband modulations. An OFDM frame is defined as a set ofconsecutive OFDM blocks. To reduce the effect of multipath delay spread,a guard time TG is introduced between each OFDM block, such thatmultipath components from one block cannot interfere with the nextblock. The guard time could contain just silence, i.e., zero-padding(ZP), or a cyclic repetition of the block, i.e., cyclic prefixing (CP).ZP determines lower transmission power and simpler transmitter structurewhen compared to CP, but potentially affects the orthogonality of thesubcarriers creating inter-carrier-interference (ICI). In the following,ZP is assumed unless otherwise specified. The expression in (2) becomes:

$\begin{matrix}{{{x(t)} = {\sum\limits_{k = 0}^{{\mathcal{F}} - 1}\;{X_{k}e^{2\;\pi\; k\; f_{s}t}{g(t)}}}},{t \in \left\lbrack {0,T_{c}} \right\rbrack}} & (3)\end{matrix}$where T_(c)=T_(B)+T_(G) is the chip time, and g(t) represents the ZPoperation

$\begin{matrix}{{g(t)} = \left\{ \begin{matrix}1 & {t \in \left\lbrack {0,T_{B}} \right\rbrack} \\0 & {otherwise}\end{matrix} \right.} & (4)\end{matrix}$The resulting data rate [bit/s] is expressed as:

$\begin{matrix}{{R = \frac{{\mathcal{F}}{\log_{2}(M)}}{T_{c}}},} & (5)\end{matrix}$where M is the modulation rate, e.g., 1 for binary phase shift keying(BPSK), that is obtained as 2^(N), N being the number of bits conveyedper symbol.

The passband transmitted signal is obtained up-converting the basebandsignal to a carrier frequency f_(c).

$\begin{matrix}{{{s(t)} = {{Re}\left\{ {\left\lbrack {\sum\limits_{k = 0}^{{\mathcal{F}} - 1}\;{X_{k}e^{2\;\pi\; k\; f_{s}t}{g(t)}}} \right\rbrack e^{{- 2}\;\pi\; f_{c}t}} \right\}}},{t \in \left\lbrack {0,T_{c}} \right\rbrack}} & (6)\end{matrix}$Note that by assuming a sampling interval T_(s), i.e., t=nT_(s), andselecting the minimum frequency spacing between subcarriers that keepsorthogonality, i.e., f_(s)=1/nT_(s), the expression in (6), except for aconstant, represents an N-point inverse discrete Fourier transform(IDFT) of the X_(k) sequence. When N is a power of two, the IDFToperation can be efficiently implemented using inversefast-Fourier-transform (IFFT) algorithms.

FIG. 1 shows a block diagram of an OFDM signal generator 10 with anencoder 20 and decoder 40. In the encoder 20, the bit stream is mappedinto a conventional modulation constellation, e.g., M-PSK and M-QAM, atsymbol mapper module 22. The serial symbol stream is converted into aparallel stream at serial-to-parallel converter 24 and fed into an IFFTmodule 26 that outputs the symbol representation in the frequencydomain. The frequency domain samples are then converted into a serialstream, and a cyclic prefix (or the zero padding) is interleaved at thebeginning of each IFFT block at parallel-to-serial converter 28. Theresulting signal is up-converted to the carrier frequency f atup-converter module 32 and transmitted. At the receiver side, thedecoder 40 down-converts the received signal to baseband atdown-converter module 42, and low-pass-filters it. The baseband filteredsignal is transformed into a parallel stream at serial-to-parallelconverter 44, fed into a fast-Fourier-transform (FFT) module 46, andre-serialized at parallel-to-serial converter 48 after discarding thecyclic prefix. The resulting symbol stream is then demapped at symboldemapper module 52 to obtain the received bit stream.

Adaptive Subcarrier Frequency-Hopping.

In some embodiments, U-OFDM can use an adaptive subcarrierfrequency-hopping that allows each transmitter to send symbols only in asubset of the available subcarriers, i.e., set of occupied subcarriersF_(o), leaving the rest empty, i.e., set of null subcarriers

_(N). The set of occupied subcarriers is selected randomly, and itchanges in each consecutive block, according to a pseudo-randomfrequency-hopping sequence (FHS), i.e., a sequence generated by seedinga random number generator with the transmitter unique ID. The number ofoccupied subcarriers for the i^(th) transmitter N_(f,i)=|F_(o,i)| can beadaptively regulated between 1 and N_(f,max)=|F| and is constant withinan OFDM frame. Moreover, the sets of occupied and null subcarrierssatisfy F_(N)=∪F_(o). The baseband OFDM block for the i^(th) transmittercan be rewritten as

$\begin{matrix}{{{x_{i}(t)} = {\sum\limits_{k \in \mathcal{F}_{o,i}}\;{X_{k}e^{2\;\pi\; k\; f_{s}t}{g(t)}}}},{t \in \left\lbrack {0,T_{c}} \right\rbrack}} & (7)\end{matrix}$Since each subcarrier carries one symbol per block the resulting datarate becomes:

$\begin{matrix}{{R\left( {\mathcal{F}_{o}} \right)} = {\frac{N_{f}{\log_{2}(M)}}{T_{c}}.}} & (8)\end{matrix}$By regulating the number of occupied subcarriers, N_(f), a transmittercan adapt its data rate based on the level of occupancy of the frequencyspectrum. The transmitter can also adapt N_(f) based on the estimatedcoherence bandwidth of the channel. By pseudo-randomly selecting the setof subcarriers in each OFDM block, the probability that communicationsfrom different transmitters completely overlap can be lowered.Overlapping subcarriers produce subcarrier collisions, thus potentialsymbol detection errors. Ideally, increasing the number of availablesubcarriers N_(f,max) may allow more transmitters to communicate in thesame channel with lower probability of subcarrier collisions. However,N_(f,max) is limited by the total available bandwidth B, which islimited because of the ultrasonic transducer characteristics, and by thecomputational power of the transmitters, i.e., the larger is N_(f,max),the higher is the computational complexity to process the digital OFDMsignals. Finally, subcarrier frequency-hopping also mitigates the effectof frequency-selective and fast fading in the intra-body channel causedby the presence of scatters and reflectors, as discussed above. Sincethe channel attenuates individually each subcarrier, the transmissionperformance highly fluctuates across different subcarriers andconsequent OFDM blocks. By pseudo-randomly selecting the set of occupiedsubcarrier in each OFDM block a transmitter can average the fadingeffect.

It will be appreciated that in some embodiments, the U-OFDM signalgenerator can use subcarriers at fixed frequencies rather than atrandomly selected subcarriers.

Adaptive Time-Hopping.

Since subcarriers N_(f) cannot be increased indefinitely, i.e., theprobability of subcarrier collision cannot be lowered indefinitely, theadaptive subcarrier frequency-hopping may not be sufficient inheavy-load scenarios. For this reason, in some embodiments, U-OFDM canalso leverage an adaptive time-hopping scheme that spreads in time OFDMblocks to further lower the probability of subcarrier collisions. Weconsider a slotted time divided in chips of duration T_(c), with chipsorganized in frames of duration T_(f)=N_(h)·T_(c), where N_(h) is thenumber of chips per frame. Each transmitter can send one OFDM block inone chip per frame, and determines in which chip to transmit based on apseudo-random time hopping sequence (THS), i.e., a sequence generated byseeding a random number generator with the transmitter's unique ID. Thebaseband OFDM block for the i^(th) transmitter and the j^(th) OFDM blockcan be rewritten as

$\begin{matrix}{{{x_{i}\left( {t,j} \right)} = {\sum\limits_{k \in \mathcal{F}_{o}^{(i)}}{X_{k}e^{2\;\pi\; k\; f_{s}t}{g(t)}{h(t)}}}},{t \in \left\lbrack {0,T_{f}} \right\rbrack}} & (9)\end{matrix}$The function h(t) represents the time-hopping spreading operation:

$\begin{matrix}{{h(t)} = \left\{ \begin{matrix}1 & {{if}\mspace{14mu} c_{j,i}} \\0 & {otherwise}\end{matrix} \right.} & (10)\end{matrix}$where {c_(j,i)} is the time hopping sequence of the i^(th) source, with0≤c_(j,i)≤N_(h)−1. The resulting data rate becomes:

$\begin{matrix}{{R\left( {N_{f},N_{h}} \right)} = {\frac{N_{f}{\log_{2}(M)}}{T_{f}} = {\frac{N_{f}{\log_{2}(M)}}{T_{c}N_{h}}.}}} & (11)\end{matrix}$By regulating the time-hopping frame length N_(h), i.e., the averageinter-block time, a transmitter can adapt its data rate, and as aconsequence modify the average radiated power and therefore the level ofinterference generated to other ongoing communications. It can beobserved that an individual user (transmitter) has little incentive toincrease its frame size, since that results in a lower achievable datarate, without any major benefit for the user itself (since the level ofinterference perceived depends primarily on the frame length of theother users, and not on its own). However, a longer time frame reducesthe interference generated to the other users. Therefore, selfish/greedyframe adaptation strategies do not work well in this context andcooperative strategies are preferred.

FIG. 2 shows an example of time-hopping strategy (top), and a combinedfrequency- and time-hopping strategy (bottom). In the pure time-hoppingstrategy, consider two users with N_(h,1)=N_(h,2)=5, transmitting oneblock B per time-hopping frame, and using time hopping sequences TH₁={3,2, 2} and TH₂={0, 5, 2}. Since the two users select the same time chipin the third time-hopping frame, a collision between the two blocksoccurs. This example can be extended by considering a combinedfrequency- and time-hopping strategy. Assume N_(f,1)=N_(f,2)=3 andN_(f,max)=8. The two users transmit one symbol S per subcarrier pertime-hopping frame. It can be observed that, although both users selectthe same subcarrier in their set of occupied subcarriers in the secondtime-hopping frame, the time-diversity introduced by the time-hoppingavoids the collision between the two symbols. Similarly, in the thirdtime-hopping frame the collision in time is avoided by leveraging of thefrequency-diversity introduced by the frequency-hopping strategy.

It will be appreciated that is some embodiments, the U-OFDM signalgenerator can use fixed time chips with a time frame. In someembodiments, the time frame can comprise a variable number of timechips, with a minimum of one time chip (i.e., no time spreading). Insome embodiments, an adaptive time-hopping scheme can be used alone,i.e., without an adaptive subcarrier frequency hopping scheme, or with afixed frequency subcarrier scheme.

Adaptive Channel Coding.

Since time and frequency hopping sequences are pseudo-randomlygenerated, collisions can still occur. In some embodiments, to mitigatethe effect of mutual interference from co-located devices, U-OFDM canimplement an adaptive channel coding that dynamically regulates thecoding rate to adapt to channel conditions and interference level. InU-OFDM, coding adaptation is performed at the subcarrier level, i.e., ineach subcarrier the coding rate is individually and independentlyadapted to combat the effect of the channel distortions that occurs inthat specific subcarrier. One embodiment uses forward error correction(FEC) functionality based on Reed-Solomon (RS) codes. RS codes arenon-binary cyclic linear block error-correcting codes used in datastorage and data transmission systems that have strong capability tocorrect both random and burst errors. An RS code can be denoted asRS(s,n,k), where s is the symbol size in bits, n is the block length andk is the message length, with k<n. An RS encoder takes k informationsymbols and adds t parity symbols to make an n symbol block. Therefore,there are t=n−k overhead symbols. On the other hand, an RS decoder isable to decode the received n-symbol block, and can correct up to t/2data symbols that may contain potential errors due to the channelfluctuation or collisions with interfering packets. The RS coding rater_(c) can be defined as the ratio between the message length and theblock length, i.e., r_(c)=k/n. Since the coding operation introducesoverhead symbols, the information rate is further reduced by a factor1/r_(c), i.e.,

$\begin{matrix}{{R\left( {N_{f},N_{h},r_{c}} \right)} = {\frac{r_{c}N_{f}{\log_{2}(M)}}{T_{c}N_{h}}.}} & (12)\end{matrix}$while the energy required for transmitting one bit is increased by afactor 1/r_(c). Note that there is a tradeoff between robustness tomulti-user interference (which increases with lower coding rate), andenergy consumption and information rate.

Alternative FEC technologies can also be used, for example convolutionalcodes that work on bit or symbol streams of arbitrary length and can beefficiently decoded with the Viterbi algorithm.

Adaptive Modulation.

In some embodiments, adaptive modulation techniques can be used, whichcomprise adapting the modulation scheme in use to the channel conditionto mitigate the effect of frequency-selective and fast fading andregulate the transmission rate. In U-OFDMA, adaptive modulationtechniques can operate at the subcarrier level, at the block level or atthe frame level. At the subcarrier level, different modulation schemesare selected for individual OFDM subcarriers in each OFDM block, toemploy higher order modulations on subcarriers with high signal-to-noiseratio (SNR), lower order modulations on subcarriers with lower SNR, andno transmission on subcarriers with very low SNR. At the block and framelevel, a single modulation scheme is used in all the subcarriers, and itcan be changed every OFDM block or frame, respectively, based on theaverage SNR in all the subcarriers.

Frame and block level adaptation offer lower complexity when compared tosubcarrier level adaptive modulation techniques, and in someembodiments, they can be a preferred choice when hardware resources area constraint in miniaturized implantable devices. However, subcarrierlevel adaptive modulation techniques offer higher granularity that canbe needed to achieve a desired communication quality of service inhighly frequency selective channels. Section 3 below discusses jointdynamic adaptation of instantaneous power, number of occupiedsubcarriers, time-hopping frame length, FEC coding rate and modulation.Finally, since the modulation can vary between OFDM frames, theinformation rate is now also a function of the modulation rate M, i.e.,R(N_(f),N_(h),r_(c),M).

Frame Synchronization.

At the receiver, OFDM frame synchronization and “time hopping”synchronization must be performed to properly decode the receivedsignal. Frame synchronization comprises finding the correct time instantcorresponding to the start of an incoming packet at the receiver, and isachieved in two steps. First, an energy collection approach identifiesany incoming frame, i.e., coarse synchronization. Once a frame isdetected, the receiver performs a fine synchronization operation thatidentifies the exact starting point of the packet. Fine synchronizationis achieved by correlating the received signal with a local copy of thepreamble, i.e., an a priori known sequence that precedes each OFDMframe. After correlating the received signal and the expected signal,the receiver identifies the starting point of the packet as the timeinstant where the correlation is maximized. The second step comprisesfinding the time-hopping sequence to hop chip-by-chip to find thetransmitted OFDM blocks. This can be achieved by seeding the randomgenerator with the same seed used by the transmitter, and thereforegenerating the same pseudo-random time-hopping sequence.

U-OFDM can use as a preamble two different sequences, i.e., a pseudonoise (PN)-sequence and a chirp sequence. The former is a binarysequence with sharp autocorrelation peak and low cross-correlationpeaks, that can be deterministically generated. Because of theirdesirable correlation characteristics, PN-sequences have strongresilience to multipath, and are well suited for ultrasonic intra-bodychannel, where reflections and scattering strongly affect the signalpropagation, as discussed above in Section 1. The chirp sequence is asinusoidal waveform whose frequency varies from an initial frequency f₀to a final frequency f₁ within a certain time T. Chirps have been usedin radars due to their good autocorrelation and robustness againstDoppler effect. In fact, a frequency-shifted chirp still correlates wellwith the original chirp, although with lower amplitude and time-shiftedpeak. This characteristic makes chirp synchronization desirable inultrasonic intra-body channels under severe Doppler effect conditions asexperienced, for example, by an ingestible pill-sized camera moving inthe digestive tract of the patient. The price paid for the Dopplerrobustness is higher cross-correlation peaks compared to PN-sequencesthat result in lower resilience to multipath.

Channel Estimation and Equalization.

As discussed in Section 1, ultrasonic intra-body communications areaffected by multipath and Doppler spread, leading to frequencyselectivity. Since by using OFDM the symbol duration in each subcarrieris long compared to channel spread, inter-symbol interference (ISI) canbe neglected in each subcarrier. However, the OFDM receiver is stronglylimited by the inter-channel interference (ICI) due to fast channelvariations within each OFDM symbol, especially if a ZP scheme is used.In some embodiments, channel estimation and equalization functionalitiescan be used to allow estimating the channel impulse response (CIR) andmitigating the distortion produced by the channel.

U-OFDM can implement both training-based and pilot-tone-based channelestimation. The training-based approach requires the presence of atraining sequence known a priori in the transmitted packet, e.g., thesynchronization preamble sequence, discussed in Section 2, to estimatethe CIR. By correlating the output of the channel, i.e., the receivedsignal, with the input, i.e., the known preamble sequence, an estimateof the time-domain CIR can be obtained. The pilot-tone-based approachestimates the channel for each OFDM block by leveraging a sequence ofpilot symbols known a priori carried by a predefined group ofsubcarriers, i.e., pilot-subcarriers. This approach is suited fortransmissions in channels that exhibit high time-variation, andtherefore require estimating the CIR in each OFDM block. U-OFDM uses theCIR estimate for equalization, e.g., zero-forcing (ZF) or maximum-ratiocombining (MRC), that aims to minimize the ICI signal distortionproduced by the channel. Finally, to further reduce the ICI distortion,frequency-offset estimation due to Doppler effect is performed byleveraging the null subcarriers.

Signal to Interference-Plus-Noise Ratio.

The signal to interference-plus-noise ratio (SINR) at the receiver oflink i averaged on all the occupied subcarriers and on a time-hoppingframe length is defined as

$\begin{matrix}{{{{SINR}_{i}\left( {P,N_{f},N_{h}} \right)} = {\sum\limits_{j}^{N_{f,i}}\;\frac{P_{i}^{(j)}g_{i,i}^{(j)}N_{h,i}T_{c}}{\eta^{(j)} + {T_{c}{\sum_{k \in I_{i}}{\frac{N_{f,k}}{N_{f,i}}P_{k}^{(j)}g_{k,i}^{(j)}}}}}}},} & (13)\end{matrix}$where P_(i) ^((j)) is the instantaneous power emitted by the i^(th)transmitter on the j^(th) subcarrier, g_(i,k) ^((j)) is the path gainbetween the j^(th) transmitter and the k^(th) receiver on the j^(th)subcarrier, and η^((j)) represents background noise energy on the j^(th)subcarrier. The set I_(i) represents the set of links whose transmitterinterferes with the receiver of link i. Note that, the expression in(13) depends on the array of instantaneous power, time-hopping framelength and number of occupied subcarrier of all the ongoingcommunications in the network, i.e., P,N_(h),N_(f), whose i^(th)elements are P_(i),N_(h,i) and N_(f,i), respectively. The termN_(f,k)/N_(f,i) is the relative number of occupied carriers between thek^(th) interferer and the i^(th) user. This ratio scales/weights theinterference effect of the k^(th) transmitter over the communication ofthe i^(th) user. When different links use different frame lengths, (13)becomes

$\begin{matrix}{{{{SINR}_{i}\left( {P,N_{f},N_{h}} \right)} = {\sum\limits_{j}^{N_{f,i}}\;\frac{P_{i}^{(j)}g_{i,i}^{(j)}N_{h,i}T_{c}}{\eta^{(j)} + {T_{c}{\sum_{k \in I_{i}}{\frac{N_{f,k}}{N_{f,i}}P_{k}^{(j)}g_{k,i}^{(j)}}}}}}},} & (14)\end{matrix}$The term N_(h,i)/N_(h,k) accounts for the level of interferencegenerated by each interferer k to the receiver of link i, i.e., thenumber of pulses transmitted by the k^(th) transmitter during the timeframe of the i^(th) user.

Note that when the node of interest increases (decreases) its framelength, N_(k,i), while the other nodes do not, no variation is expectedin the SINR (there is in fact a slight increase (decrease) in the SINR,which can be neglected under high SNR conditions,η«Σ_(k∈1i)P_(k)g_(ki)). Finally, when the frame length of theinterfering nodes is increased (decreased), the SINR increases(decreases). On the other hand, when the interfering nodes increase(decrease) their number of occupied subcarriers, the SINR decreases(increases), since the probability of subcarrier collision increases(decreases).

As can be observed in (14), modulation and FEC coding rate do not affectthe average SNR measured at the receiver. Instead, these two parametersaffect the relation between bit-error-rate (BER) experienced at thereceiver and SINR. Lower FEC coding rate means more overhead symbols,which potentially lower the BER after decoding. Thus, for a givenminimum BER threshold, lower FEC coding rates require lower minimumSINR. On the other end, for a given BER requirement, increasing themodulation order requires a higher minimum SINR level to achieve thatBER. For these reasons, the minimum SINR requirement, i.e., SINR_(min),is expressed as a function of the FEC coding rate and the modulationorder, i.e., SINR_(min)(r_(c),M).

3. MAC and Rate Adaptation

In some embodiments, U-OFDM medium access control protocol and rateadaptation schemes can be provided. Based on the discussion so far,there is a tradeoff between (i) resilience to interference and channelerrors, (ii) achievable information rate, and (iii) energy efficiency.Thus, medium access control and rate adaptation strategies can beprovided that find optimal operating points along efficiency-reliabilitytradeoffs. Rate-maximizing adaptation strategies are discussed inSection 3.1. Energy-minimizing strategies are discussed in Section 3.2.

3.1 Distributed Rate-Maximizing Adaptation

The objective of the rate-adaptation scheme under consideration is tolet each active communication maximize its transmission rate byoptimally selecting the instantaneous power, the number of occupiedsubcarrier, the time-hopping frame length, the FEC coding rate andmodulation, based on the current level of interference and channelquality measured at the receiver and on the level of interferencegenerated by the transmitter to the other ongoing communications. Adecentralized ultrasonic intra-body area network is considered, with

being the set of |

| existing connections. Note that there are no predefined constraints onthe number of simultaneous connections |

|. Denote by N_(h,max), r_(c,max), M_(max) and P_(max), the maximumtime-hopping frame length, the maximum coding rate, the maximummodulation rate and the maximum instantaneous power supported,respectively. Thus0<N _(f,i) ≤N _(f,max) ,∀i∈

,N _(f)∈

,  (15)0<N _(h,i) ≤N _(h,max) ,∀i∈

,N _(h)∈

,  (16)r _(c,i) ∈{r _(c,i) ,r _(c,2) , . . . ,r _(c,max) }∀i∈

  (17)M _(i) ∈{M1,M2, . . . ,M max}∀i∈

,  (18)0≥P _(i) ≤P _(max) ∀i∈

,  (19)where

is the set of natural numbers. According to the transmission schemediscussed in Section 2, each node i transmits at a rate R^((i))expressed as in (12) and each receiver experiences an SINR expressed asin (14). Each node has a minimum data rate requirement,R _(i)(N _(f,i) ,N _(h,i) ,r _(c,i) ,M _(i))≥R _(min),  (20)and a minimum SINR requirement,SINRi(P _(i) ,N _(f,i) ,N _(h,i))≥SINR_(min)(r _(c,i) ,M _(i))  (21)

The receiver is in charge of estimating interference and finding theinstantaneous power, number of occupied subcarriers, time-hopping framelength, FEC coding rate and modulation that maximize the systemperformance. Accordingly, denote instantaneous power, the number ofoccupied subcarrier, the time-hopping frame length, the FEC coding rateand modulation selected by the receiver of the connection r, as P_(r),N_(f,r),N_(h,r),r_(c),M_(r).

The objective of each user is to locally optimize the information rateof the connection by solving the following problem:find P _(r) ,N _(f,r) ,N _(h,r) ,r _(c,r) ,M _(r)  (22)that maximize R _(r),(N _(f,r) ,N _(h,r) ,r _(c,r) ,M _(r))  (23)subject to R _(r),(N _(f,r) ,N _(h,r) ,r _(c,r) ,M _(r))≥R _(min)  (24)SINR_(r)(P _(r) ,N _(f,r) ,N _(h,r))≥SINR_(r,min)(r _(c,r) ,M_(r))  (25)SINR_(i)(P _(i) ,N _(f,i) ,N _(h,i))≥SINR_(i,min)(r _(c,i) ,M _(i))∀i∈

_(r),  (26)where

_(r) is the set of the connections interfering with the r^(th)connection. The constraints on the maximum frame and code length in(15), (16), (17), (18) and (19) are also implicitly considered.3.2 Distributed Energy-Minimizing Rate Adaptation

Rate adaptation has the objective of reducing the energy consumption ofU-OFDM. Section 2 mentioned that adaptive frequency and time hoppingtechniques, adaptive FEC coding and adaptive modulation affect theenergy consumption of the transmitting device. For this reason,energy-related metrics are introduced that make the dependence of theenergy consumption on number of occupied subcarriers, time-hopping framelength, FEC coding and modulation explicit.

E_(b), the energy per bit is defined as:E _(b) =P·T _(c)/(r _(c) ·N _(f)·log₂(M))  (27)

E_(s), the average power radiated per second is defined as:E _(s) =P/(N _(h)).  (28)

The energy per bit is a function of the inverse of the FEC coding rate,number of occupied subcarriers and number of bits transmitted persymbol. Higher coding rate, number of occupied subcarrier and modulationorder decrease the energy consumption. The average power emitted persecond is a function of the inverse of the time-hopping frame length andhence of the number of OFDM blocks transmitted per second.

Energy-Minimizing Rate Adaptation.

Based on this model, a rate adaptation strategy is provided where theobjective is to minimize (i) the energy per bit, E_(b), or (ii) theaverage energy emitted per second E_(s). The problem can be cast asfinding the optimal frame length and the optimal spreading code lengththat minimize E_(h) (and/or E_(s)) while meeting the minimum SINRconstraints and keeping the data rate over a given threshold. Theproblem is formally expressed below.

Find P_(r),N_(f,r),N_(h,r),r_(c,r)M_(r),

that minimizes E_(b)(P_(r),r_(c,r),N_(f,r),M_(r)) (or E_(s)(N_(h,r)))

subject to Equations (24, (25) and (26).

3.3 Medium Access Control Protocol

In some embodiments of U-OFDM, distributed medium access controlcoordination can be achieved by exchanging information on logicalcontrol channels, while data packets are transmitted over logical datachannels. Unicast transmissions between a transmitter TX and a receiverRX are considered, as follows.

When TX needs to transmit a packet, it first needs to reserve adedicated channel to RX. The connection is opened through the commoncontrol channel using a two-way handshake procedure. In U-OFDM thecontrol channel can be implemented using two different alternativeapproaches, fixed control channel and random control channel. In thefixed control channel approach, a fixed number of preassignedsubcarriers are allocated to transmit and receive control information.In the control subcarriers the communication follows a uniquetime-hopping sequence known and shared by all network devices. All thenodes listen to the fixed control channel and wait for a request from atransmitting node. The control channel is accessed through a contentionphase.

In the random control channel approach, control channel is implementedin a frequency-hopping fashion, i.e., the control channel subcarrierallocation changes pseudo-randomly in time. Synchronization between thetransmitting and receiving nodes is possible by guaranteeing that thetransmitter use all the channels in a fixed period of time, so that thereceiver can then find the transmitter channel by picking a randomchannel and listening for valid data on that channel.

In the two-way handshake procedure, TX sends a Request-to-Transmit (R2T)packet to RX, which contains its own ID. If RX is idle, aClear-to-Transmit (C2T) control packet is sent back to TX. In case offailure and consequent timer expiration, TX will attempt a newtransmission after a random backoff time, for a maximum of NR times.After receiving the C2T packet, the transmitter switches to a dedicatedchannel by computing its own frequency- and time-hopping sequence byseeding a pseudo-random sequence generator with its own ID. As aconsequence, both TX and RX leave the common channel and switch to adedicated channel. The receiver RX computes the optimal transmissionstrategy, i.e., number of occupied subcarrier, time-hopping framelength, FEC coding rate and modulation, as discussed in Section 2. Thisinformation is piggybacked into ACK or NACK packets.

Once the communication has been established, RX does not leave thecommon control channel. Instead, it keeps “listening” to both thededicated and common control channels at the same time. In the dedicatedcontrol channel, RX sends to TX the optimal strategy information to beused for the next transmission. In the common control channel, RXexchanges with other co-located receivers information on the level oftolerable interference.

3.4 Network Configuration

U-OFDM can internetwork implantable devices in master/slave (M/S) orpeer-to-peer (P2P) configurations. Both configurations can coexist inthe same intra-body network, referred to as hybrid configurations.

Master-Slave Configuration.

In the M/S configuration, one node takes the role of master, i.e.,network coordinator, while the remaining nodes operate as slaves. Inthis scenario, the network control is concentrated on a master node.Network access of the slave node is deterministically regulated througha polling mechanism, e.g., the master node has complete control overchannel access, while each slave node is granted access to the medium ina round-robin fashion.

Peer-to-Peer Configuration.

In the P2P configuration, all the network nodes are treated as peers. Inthis configuration, network access can be regulated as discussed inSection 3.3 through fixed or random control channel.

The communication system and method for transmitting data ultrasonicallythrough biological tissue can be advantageously implemented among anetwork comprising a plurality of nodes 110 in which at least a portionof the nodes are implantable within a body 120. See FIG. 3. In someembodiments, at least one of the implantable nodes, a first node,comprises an ultrasonic transducer and a transmitter, and a second node,which can be implantable within the body or disposable outside the body,comprises an ultrasonic receiver. The transmitter at the first nodeincludes an orthogonal frequency division multiplex (OFDM) signalgenerator operative to encode an input information bit stream onorthogonal subcarriers for transmission as an ultrasonic signal throughthe body to the ultrasonic receiver at the second node. The ultrasonicreceiver at the second node is operative to decode the ultrasonic signalreceived from the first node to recover the information bit stream. Insome embodiments, all of the nodes are implantable in a body. In someembodiments, the nodes are operable to support an ultrasoniccommunication data rate of at least 28 Mbit/s. A data rate greater than28 Mbit/s is particularly suitable for ultrasonic communications inintra-body networks for applications such as neural data recording orwireless endoscopic pills.

Each node can include a combination of hardware, software, and/orfirmware that allows the system to perform the various tasks asdescribed herein. The nodes can be implemented as microprocessor-basedcomputing devices, microcontroller-based computing devices, and thelike. The computing devices can include one or more processors andmemory that cooperate with an operating system to provide basic andsupport functionality for an applications layer and other processingtasks. Various types of processing technology can be used, including asingle processor or multiple processors, a central processing unit(CPU), multicore processors, parallel processors, or distributedprocessors. Additional specialized processing resources, such asmathematical processing capabilities, can be provided to perform certainprocessing tasks. Other hardware components and devices can interfacewith the computing device. As used herein, the term “transceiver” caninclude one or more devices that both transmit and receive signals,whether sharing common circuitry, housing, or a circuit board, orwhether distributed over separated circuitry, housings, or circuitboards, and can include a transmitter-receiver.

The computing device includes memory or storage, which can be accessedby a system bus or in any other manner. Memory can store control logic,instructions, and/or data. Memory can include transitory memory, such ascache memory, random access memory (RAM), static random access memory(SRAM), main memory, dynamic random access memory (DRAM), and memristormemory cells. Memory can include storage for firmware or microcode, suchas programmable read only memory (PROM) and erasable programmable readonly memory (EPROM). Memory can include non-transitory or nonvolatile orpersistent memory such as memory chips and memristor memory cells. Anyother type of tangible, non-transitory storage that can provideinstructions and/or data to a processor can be used in theseembodiments.

The communication system and method can be used in a variety ofapplications that require transmission of data through biologicaltissue. For example, the system and method can be used with implantablesensors, such as cardiac rhythm monitors, pulse monitors, blood pressuresensors, glucose sensors, drug pump monitors, neurological sensors,motion sensors, gyroscopes, accelerometers, sleep sensors, REM sleepduration sensors, still cameras, or video cameras, to transmitphysiological data that is captured by the implantable sensors to agateway outside of the body or to other implanted devices within thebody. The system can be used to communicate actuation commands to andobtain data from implantable devices such as drug delivery systems ordrug pumps, heart stimulators, pacemakers, neuromuscular electricalstimulators, and bone growth stimulators. For example, the system andmethod can be used to obtain data from a glucose monitor in a diabeticpatient and to communicate instructions to and obtain data from aminiaturized, under-the-skin insulin pump. As another example, thesystem and method can be used with pill-sized ingestible cameras thatare used to monitor the digestive tract of a patient. In someembodiments, the system and method can implemented as an endoscopicpill. The system can be used with a human body or with a non-humananimal body.

In certain embodiments, a node can include a sensor for one or morebiomolecules. Examples of such biomolecules include peptides,oligopeptides, polypeptides, proteins, glycoproteins, antibodies,antigens, nucleic acids, nucleotides, oligonucleotides, polynucleotides,sugars, disaccharides, trisaccharides, oligosaccharides,polysaccharides, lipids, glycolipids, proteolipids, cytokines, hormones,neurotransmitters, metabolites, glycosaminoglycans, and proteoglycans.In certain embodiments, a node can include a sensor for one or morepharmaceutical agents or pharmaceutical formulation ingredients. Incertain embodiments a node can include a sensor for a dissolved gas orion, or for pH, ionic strength, or osmolarity.

EXAMPLES

An OFDM signaling scheme for intra-body ultrasonic communications wasprototyped and tested to determine achievable data rates throughsynthetic phantoms mimicking the ultrasonic propagation characteristicsof biological tissues. In a first set of experiments, the focus was onrealizing real-time implementation of the OFDM physical layer andevaluating its performance in terms of BER for different transmissionpower levels. In a second set of experiments, the focus was onmaximizing the data rate of the proposed scheme and evaluating resultswith offline processing.

Example 1

An OFDM signaling scheme was designed using a packet format having Nconsecutive OFDM blocks. To reduce the effect of multipath delay spread,a zero-padding (ZP) technique was used, which is based on padding zerosafter each OFDM block to act as a guard interval. ZP was chosen overother alternatives such as cyclic-prefixing (CP) because of itsenergy-efficiency. In the proposed packet format, each OFDM block wasdesigned to have K subcarriers, which were assigned with three differentroles: (i) KD subcarriers were designated as data subcarriers forallocating data symbols, where each data symbol was a modulated versionof information bits with different gray-coded modulation schemes (i.e.,Binary-Phase-Shift-Keying (BPSK), Quadrature-Phase-Shift-Keying (QPSK),8-PSK, 8-Quadrature-Amplitude-Modulation (8-QAM), 16-QAM, 32-QAM,64-QAM, 128-QAM); (ii) K_(P) subcarriers were charged as pilotsubcarriers for carrying symbols that were known both by the transmitterand the receiver. Pilot subcarriers were equally spaced within the OFDMblock and carried symbols that were mapped with BPSK for decreasing thecomplexity of the receiver algorithms. The pilot subcarriers wereexploited for performing channel estimation, block-level (fine)synchronization, and supporting Doppler scale estimation; (iii) KN nullsubcarriers were assigned to be used for Doppler scale estimation.Moreover, the packet format included a preamble, i.e.,Pseudorandom-Noise (PN) sequence, preceding each packet for using inpacket detection and coarse-synchronization operations.

FIG. 4 illustrates a block diagram of the prototyped OFDM scheme. At thetransmitter side, the information bits were mapped into symbols based onthe selected modulation scheme. The generated serial symbol stream wasthen converted into a parallel stream in accordance with the subcarrierpositions along with pilot and null symbols. Consequently, the parallelstream was fed into an IFFT block, which output the symbolrepresentations in the time domain as a serial stream. Later on, thezero-padding operation was performed to generate ZP-OFDM blocks. Theformed ZP-OFDM blocks were fed into an operational block, where theywere transformed into the packet format with the addition of a preambleblock. Finally, the resulting stream was up-converted to the carrierfrequency and transmitted. At the receiver side, first the receivedsignal was down-converted to baseband and a low-pass filter (LPF) wasused for filtering out the out-of-band noise and interference. Thefiltered baseband signals were then fed into synchronization blocks,where the correlation properties of the PN sequence were exploited toperform frame detection and coarse-synchronization. Following thecoarse-synchronization, the OFDM frame was partitioned into individualOFDM blocks by performing the block-level synchronization by using thepilot symbols. The partitioned OFDM blocks were then converted intoparallel streams to be fed into the FFT block, where they were convertedto serial streams in the frequency domain. Later on, the frequencydomain streams were passed to a block, where the Doppler scale wasestimated and compensated accordingly. The Doppler-compensated OFDMblocks were fed into a channel estimation block, which performedpilot-tone based channel estimation and channel equalization. Finally, areceiver block that incorporated a Zero-Forcing receiver mappedequalized symbols onto the bits.

Example 2

A custom software-based radio (SDR)-based node for testing ultrasoniccommunication schemes was designed and used. The testbed included two ofthese SDR-based nodes, which supported an ultrasonic communication linkthrough an ultrasonic phantom that mimicked the acoustic propagationproperties of biological tissues with high fidelity. The customSDR-based nodes included (i) a USRP N210, (ii) a host Linux-PC, (iii) apower amplifier/a voltage preamplifier, (iv) an electronic switch, and(v) an ultrasonic transducer. FIG. 5 depicts the hardware architectureof the node.

A Universal Software Radio Peripheral, USRP N210, a Field ProgrammableGate Array (FPGA) based, SDR platform, commercially available from EttusResearch, was selected due to its low cost and wide adoption in academiaand industry. The USRP N210 included a motherboard and twodaughterboards. Specifically in the node design, LFTX and LFRXdaughterboards were selected, which covered a frequency band of (DC—30MHz) that enabled a half-duplex transceiver operating in the ultrasonicfrequency ranges of interest for this application. The motherboard actedas the main processing unit, which was equipped with ananalog-to-digital-converter (a dual 100 MS/s 14-bit ADC) and adigital-to-analog-converter (a dual 400 MS/s 16-bit DAC), that were bothcontrolled by a 100 MHz master clock, and an FPGA unit (Xilinx Spartan3A-DSP3400). The sampling rate of incoming digital samples (from ADC)and outgoing samples (to DAC) was fixed at 100 MS/s, while the FPGAdigitally interpolated/decimated the sample stream to match the hardwaresampling rate to the rate requested by the user. In USRP, high ratebaseband signal processing is typically performed in a host-PC that isconnected to the USRP through a Gigabit Ethernet (GigE) connection or inthe FPGA.

The host machine was the processing unit that typically handled highrate baseband signal processing functionality. It can be either adesktop/laptop computer or a computer-on-module, e.g., Gumstix,Raspberry Pi. To implement signal processing functionalities, opensource GNU Radio was used to implement the software radio. GNU Radio wasused (i) to drive the USRP operations from the host-PC, (ii) as well asto implement signal processing operations in combination with MATLABscripting language. GNU Radio provided a plethora of signal processingblocks that are implemented in C++ and that can be leveraged to rapidlyimplement a wide range of wireless communication schemes. In thetestbed, the GNU Radio framework was selected because of its large setof available building blocks and because of the capability of creatingnew custom blocks easily for implementing customized communicationschemes. Also, the receiver functionalities of GNU Radio blocks andMATLAB scripting language were combined for experiments.

The testbed used amplifiers to enhance the communication range andperformance of the ultrasonic link. At the transmitter side, a COTSpower amplifier (PA), LZY-22+, available from Mini-Circuits, was used.This PA was capable of providing a gain of 43 dB across the operatingfrequency of 0.1 to 200 MHz and leveraged to amplify the output power ofthe LFTX daughterboard (i.e., 2 mW). At the receiver side, a COTSLow-Noise Amplifier (LNA), ZFL-1000LN+, available from Mini-Circuits,was used. This LNA offered a low-noise figure of 2.9 dB. To enablefull-duplex operations with a single ultrasonic transducer on a timedivision basis, the node design incorporated a COTS electronic switch,ZX80-DR230+, available from Mini-Circuits, which offered a low insertionloss with a very high isolation over the frequency range of 0 to 3 GHz.The electronic switch was driven by leveraging the General PurposeInput/Output (GPIO) digital pins available on the LFTX and LFRXdaughterboards.

Ultrasonic transducers were used that were capable of generating anddetecting ultrasonic waves over a range of frequencies of interest tothis application. Typically, ultrasonic transducers are based on thepiezoelectric effect, which enables two-way conversion betweenelectrical and ultrasonic energy. The main factor in determining theultrasonic transducer to be used in the system was the operatingfrequency. As noted above, while increasing the operating frequencyallows the size of the transducer to be decreased, this results inhigher signal attenuation. Therefore, to be able to operate over thelinks on the order of several cm, the operating frequency should notexceed 10 MHz. Moreover, most biomedical sensing applications requiredirectional transducers. Considering these factors, small-sizedirectional ultrasonic transducers were selected that were able tosupport lowest possible frequencies and large signal bandwidths. Hence,standard immersion ultrasonic transducers, Ultran WS37-5, available fromThe Ultran Group, were used. These transducers offered approximately anominal bandwidth of 5 kHz at the central frequency of 5 kHz.

Example 3

To emulate the intra-body ultrasonic communication channel with highfidelity, ultrasonic phantoms were used. The ultrasonic phantoms weretissue-mimicking materials (tissue substitutes) that had the sameacoustic propagation characteristics of human tissues, e.g., soundspeed, density, and attenuation. The testbed was based on anoff-the-shelf human-kidney phantom, available from Computerized ImagingReference Systems, Inc., immersed in a background water-based gel thathad dimensions of 10 cm×16 cm×20 cm The background gel had the samedensity and sound speed as the kidney, which minimized the possiblereflections and retractions and accordingly can be consideredacoustically transparent. The acoustic characteristics of the phantomare summarized in Table 1.

TABLE 1 Acoustic Characteristics of the Ultrasonic Phantom Tissue Speed,ν Attentuation, α Density, ρ Background Gel 1550 m/s <0.1 dB/cm 1020Kg/m³ Kidney 1550 m/s 2 dB/cm @ 5 MHz 1030 Kg/m³

FIGS. 6 and 7 illustrate a testbed setup with two SDR-based ultrasonicnodes communicating through a human-kidney phantom.

Example 4

A first set of experiments was aimed at achieving a real-timeimplementation of the communication scheme described in Example 1 andusing the testbed setup of Examples 1-3 and FIGS. 6 and 7. To obtainthis, all the processing blocks using GNU Radio on the host machine weredesigned and implemented. However, the sampling rate and accordingly thedata rate of this implementation were limited by two main factors: (i)the link capacity of the GigE connection between the host machine andthe USRP that limited the maximum achievable sampling rate, i.e., 25MS/s. When this sampling rate was exceeded, the GigE connection startedexperiencing network packet drops that caused loss of digital samples;and (ii) purely software implementations (GNU Radio) of digital signalprocessing blocks introduced high processing latency, which eventuallyoverloaded the host machine when operating at high sampling rates, i.e.,typically greater than 10 MS/s. Therefore, if the host machine were notcapable of processing the data as fast as the sampling rate, theinternal buffers that stored digital samples overflowed and caused lossof digital samples. To overcome this limitation, considering theprocessing time of the implementation, the sampling rate was limited to781 kS/s, which corresponded to a bandwidth of 390 kHz. A carrierfrequency of 5 MHz was used, which matched with the center frequency ofthe ultrasonic transducers. A packet structure was defined thatincorporated 32 OFDM blocks, each of them including 2048 total number ofsubcarriers and 128 pilot subcarriers. Each data subcarrier carried aninformation symbol that was mapped either with BPSK, QPSK, 8QAM, 16QAM.As a note, specifically for this set of experiments, the power amplifierwas not incorporated in the testbed setup.

FIG. 8 shows BER versus SNR performance for different modulation schemesand accordingly different data rates. Using a higher modulation schemeincreased the data rate at the expense of BER performance. Theultrasonic communication scheme was able to support data rates up to 1.3Mbit/s at BER lower than 10⁴ with real-time processing on the hostmachine.

Example 5

In a second set of experiments, the objective was to maximize the datarate of the ultrasonic communication scheme over the testbed setup. Asexplained in Example 4, a purely software implementation (GNU Radio) ofthe communication scheme limits the maximum data rate that can bereached. While it is possible to overcome this limitation by migratingsoftware processing from the host machine to the FPGA to effectivelyspeed up data processing and reduce the computational load on the hostmachine, such an approach was beyond the scope of this set ofexperiments. To that end, offline experiments were performed, in whichGNU Radio was used only for recording the transmitted data whileprocessing it offline with MATLAB.

The sampling rate was increased to 10 MS/s, which translated into abandwidth of 5 MHz. Similar to the previous set of experiments, acarrier frequency of 5 MHz was used. A packet structure was used thatincluded 32 OFDM blocks. Each of them had 32768 total number subcarriersand 2048 pilot sub-carriers. Each data subcarrier conveyed aninformation symbol that was mapped with higher order modulation schemes,i.e., 8QAM, 16QAM, 32QAM, 64QAM, 128QAM.

Table 2 shows BER versus SNR performance for different modulationschemes and accordingly different data rates. Data rates of 28.12 Mbit/swere reached with a BER performance of 10⁻¹ while data rates up to 20.15Mbit/s were achievable with a BER lower than 3×10⁻². As a note, in thisspecific set of experiments, forward-error-correction (FEC) coding,e.g., convolutional codes, that can trade off BER performance for datarate, were not considered.

TABLE 2 BER versus SNR Results for Different Modulation SchemesModulation BER ν SNR [dB] Data Rate [Mbit/s]  8QAM 1.9 × 10⁻⁴ 17 12.0916QAM 5.8 × 10⁻³ 18 16.12 32QAM 3.1 × 10⁻² 13 20.15 64QAM 8.0 × 10⁻² 1924.18 128QAM  1.3 × 10⁻¹ 20 28.12

As used herein, “consisting essentially of” allows the inclusion ofmaterials or steps that do not materially affect the basic and novelcharacteristics of the claim. Any recitation herein of the term“comprising,” particularly in a description of components of acomposition or in a description of elements of a device, can beexchanged with “consisting essentially of” or “consisting of.”

It will be appreciated that the various features of the embodimentsdescribed herein can be combined in a variety of ways. For example, afeature described in conjunction with one embodiment may be included inanother embodiment even if not explicitly described in conjunction withthat embodiment.

The present invention has been described in conjunction with certainpreferred embodiments. It is to be understood that the invention is notlimited to the exact details of construction, operation, exact materialsor embodiments shown and described, and that various modifications,substitutions of equivalents, alterations to the compositions, and otherchanges to the embodiments disclosed herein will be apparent to one ofskill in the art.

What is claimed is:
 1. A system for transmitting data ultrasonicallythrough biological tissue comprising: a network comprising a pluralityof nodes, at least a portion of the nodes implantable within a body; afirst node implantable in the body and comprising an ultrasonictransducer and a transmitter, and a second node comprising an ultrasonicreceiver and transmitter; the transmitter at the first node including anorthogonal frequency division multiplexing (OFDM) signal generatoroperative to encode an input information bit stream on orthogonalsubcarriers for transmission as an ultrasonic signal through the body tothe ultrasonic receiver at the second node, the transmission adapted toa level of interference between the portion of the nodes implantablewithin the body; and the ultrasonic receiver at the second nodeoperative to decode the ultrasonic signal received from the first nodeto recover the information bit stream; wherein the transmitter at thefirst node is operative to open communication to the receiver at thesecond node, and the receiver is operative to transmit to thetransmitter an optimal transmission strategy adapted to the level ofinterference, the transmission strategy including a number of occupiedsubcarriers, a time-hopping frame length, instantaneous power value, aforward error correction coding rate, and modulation rate.
 2. The systemof claim 1, wherein the OFDM signal generator is operative to generate abaseband modulated signal as a sum over a number of subcarriers of thesymbols to be transmitted as a function of a frequency spacing betweenthe subcarriers for a time block of a given duration.
 3. The system ofclaim 2, further comprising introducing a guard time between timeblocks, wherein the guard time comprises silence or a repetition of thetime block.
 4. The system of claim 2, further comprising a symbol mapperto map the bit stream into a constellation of a modulation scheme. 5.The system of claim 4, wherein the modulation scheme comprisesM-phase-shift-keying or M-quadrature-amplitude-modulation.
 6. The systemof claim 1, wherein the OFDM signal generator comprises: a serial toparallel convertor to convert the input information bit stream into aplurality of parallel data strings, an inverse Fourier transformer togenerate a frequency domain representation of the input information bitstream, a parallel to serial converter for converting the frequencydomain representation into a serial stream, and an up-converter toconvert the signal to a carrier frequency for transmission through thebiological tissue.
 7. The system of claim 1, wherein the receivercomprises: a down-converter to convert the received signal to a basebandsignal, a serial to parallel converter to convert the baseband signalinto a plurality of parallel data strings, a Fourier transformer; and aparallel to serial converter to convert the parallel data strings into aserial data string.
 8. The system of claim 1, wherein the OFDM signalgenerator is operative to send symbols on a set of occupied subcarrierscomprising a subset of available subcarriers, wherein the occupiedsubcarriers are fixed or selected randomly and change in consecutiveblocks within a frame.
 9. The system of claim 8, wherein the occupiedsubcarriers are selected by a pseudo-random frequency-hopping sequencegenerated by seeding a random number generator with an identificationunique to the transmitter.
 10. The system of claim 1, wherein the OFDMsignal generator is operative to send symbols in blocks at fixed orrandomly selected time chips within a time frame.
 11. The system ofclaim 1, wherein the OFDM signal generator is operative to provideforward error correction.
 12. The system of claim 1, wherein the OFDMsignal generator is operative to provide one or more modulationtechniques at a subcarrier level, a block level, or a frame level. 13.The system of claim 12, wherein the modulation technique is selected tooptimize a data rate as a function of one or more of a number ofoccupied subcarriers, a number of time chips per time frame, an errorcorrection coding rate, and a modulation rate.
 14. The system of claim1, wherein the receiver is operative to detect an incoming frame fromthe transmitter and to identify a starting point of a packet, whereinidentifying the starting point comprises correlating the receivedultrasonic signal with a local copy of a preamble preceding each OFDMframe.
 15. The system of claim 1, wherein the receiver is operative todetermine one or more of a signal to interference-plus-noise ratio as afunction of instantaneous power, time-hopping frame length, and numberof occupied subcarriers.
 16. The system of claim 1, wherein the receiveris operative to maximize a transmission rate between the transmitter andthe receiver by selecting one or more of an instantaneous power, anumber of occupied subcarriers, a time-hopping frame length, a forwarderror correction coding rate and a modulation rate based on a level ofinterference and channel quality measured at the receiver and on a levelof interference generated by the receiver in communications to othernodes.
 17. The system of claim 1, wherein the receiver is operative todetermine one or more of the instantaneous power value, the number ofoccupied subcarriers, the time-hopping frame length, the forward errorcorrection coding rate and the modulation rate to maximize a data rate.18. The system of claim 17, wherein the receiver is further operative tomaximize the data rate subject to a signal to interference-plus-noiseratio per node being above a minimum value and a data rate per nodebeing above a minimum value.
 19. The system of claim 1, wherein thereceiver is further operative to determine an energy rate, the energyrate comprising an energy per bit or an average power radiated persecond.
 20. The system of claim 19, wherein the receiver is furtheroperative to minimize the energy rate subject to a signal tointerference-plus-noise ratio per node being above a minimum value and adata rate per node being above a minimum value.
 21. The system of claim1, wherein the transmitter is operative to open communication to thereceiver on a common control channel using a two-way hand-shakeprocedure, and, after receiving a clear-to-transmit signal from thereceiver, to transmit on a dedicated channel to the receiver afrequency-hopping sequence and a time-hopping sequence, and the receiveris operative to transmit to the transmitter the optimal transmissionstrategy.
 22. The system of claim 1, wherein the first implantable nodefurther comprises one or both of a sensor operative to sense one or morebiological parameters or an actuator.
 23. The system of claim 1, whereinthe first node and the second node are operable at a data rate of atleast 28 Mbit/s.
 24. A method for transmitting data ultrasonicallythrough biological tissue comprising: in a network comprising aplurality of nodes, at least a portion of the nodes implanted within abody including a first node implanted in the body and comprising anultrasonic transducer and a transmitter, and a second node comprising anultrasonic receiver and transmitter; at the first node, encoding aninformation bit stream on orthogonal subcarriers using an orthogonalfrequency division multiplexing (OFDM) modulation scheme fortransmission adapted to a level of interference between the portion ofthe nodes implantable within the body; at the first node, openingcommunication to the receiver at the second node: at the second node,determining an optimal transmission strategy adapted to the level ofinterference, the transmission strategy including a number of occupiedsubcarriers, a time-hopping frame length, instantaneous power value, aforward error correction coding rate, and modulation rate, andtransmitting the optimal transmission strategy to the first node: at thefirst node, transmitting the encoded signal through biological tissueaccording to the transmission strategy: and at the second node,receiving the encoded signal and decoding the signal to recover theinformation bit stream.
 25. The method of claim 24, wherein the firstnode and the second node are operable at a data rate of at least 28Mbit/s.